The invention relates to a cathode-ray tube for displaying coloured pictures, comprising in an evacuated envelope means to generate a number of electron beams, a display screen comprising a large number of regions luminescing in different colours, and colour selection means constituted by a ferromagnetic plate having a plurality of apertures which associate each electron beam with luminescent regions of one colour, said plate being magnetized in such manner that a magnetic quadrupole field is formed in each aperture.
Such a cathode-ray tube of the post-focussing type is known from U.S. Pat. No. 4,135,111. The object of post-focussing is to increase the brightness of the displayed picture by increasing the transmission of the colour selection means. In tubes without post-focusing a very large part, for example 80 to 85%, of the electrons are intercepted by the so-called shadow mask. By using post-focusing the apertures in the colour selection means can be enlarged since as a result of the focusing in the apertures the electron spots on the screen are considerably smaller than the apertures so that nevertheless sufficient space exists between the electron spots of the various electron beams.
The electron lens which is formed in the apertures of the shadow mask of the known type is a magnetic quadrupole lens. Since the magnetic field is perpendicular to the electron path, quadrupole lenses are comparatively very strong as a result of which a comparatively small magnetic field will suffice. The characteristic that a quadrupole lens focuses in one direction and defocuses in the direction at right angles thereto is in principle no objection if all quadrupoles have the same orientation. Therefore the luminescent regions of the display screen preferably have the shape of substantially parallel strips the longitudinal direction of which is substantially parallel to the defocusing direction of quadrupole lenses.
Two different alloys have been suggested for the material of the foil of permanent magnetic material which forms the shadow mask of the known tube, namely an alloy which comprises 20% by weight of Fe, 20% by weight of Ni and 60% by weight of Cu, and one which in addition to Fe comprises 27% by weight of Cr, 15% by weight of Co, 1% by weight of Nb and 1% by weight of Al. However, insofar as said alloys already satisfy the requirements which, as regards magnetic properties, are imposed for such an application
H.sub.c .gtoreq.20 kA/m PA1 B.sub.r .gtoreq.0,5 Tesla PA1 T.sub.c &gt;450.degree. C. PA1 25-27% by weight of Cr PA1 10-11% by weight of Co,
they do not, however, satisfy the mechanical requirements which are imposed to realise a shadow mask having a desired curvature whether they are in the form of a plate or a foil. A plastic elongation of 8% at least is required for this latter.